1. Field of the Invention
The present invention relates to a locally pressurizing type injection molding machine.
2. Description of the Prior Art
Conventional locally pressurizing type injection molding machines pressurize a local portion of a resin within a cavity, thereby eliminating the need for the boring of a thin plate, the finishing of a cut surface or the like.
FIG. 1 is a view illustrating operation of the prior art locally pressurizing type injection molding machine. FIG. 1(a) illustrates a molding step; FIG. 1(b) illustrates a punch vibrating step; and FIG. 1(c) illustrates a vibration punching step.
Referring to FIG. 1, the prior art locally pressurizing type injection molding machine comprises a stationary die 1 mounted on a stationary platen, a movable die 2 mounted on a movable platen (not shown), a cavity 3 defined between the stationary and movable dies 1 and 2, a sprue 4 through which a resin is injected from an injection nozzle (not shown), a gate 5 located between the sprue 4 and the cavity 3, a stationary punch 6 disposed on the stationary die 1, and a movable punch 7 disposed on the movable die 2.
The stationary and movable punches 6 and 7 are vibrated by vibrating devices (not shown) disposed within the stationary and movable dies 1 and 2, respectively, during a cooling step which follows the completion of a pressure maintaining step in an injection molding cycle. The stationary and movable punches 6 and 7 are previously disposed at locations where they contact a portion of a molding and hence, as shown in FIG. 1(a), heat due to shear stress is generated in such portion (e.g., a boring portion or a gate 5) between the stationary and movable punches 6 and 7 by vibration of the movable punch 7, thereby softening the resin.
When the movable punch 7 is pushed out to the right, as shown in FIG. 1(b), while vibrating the stationary and movable punches 6 and 7, the softened resin is cut, as shown in FIG. 1(c). In this case, the cut along the movable punch 7 is finished and cooled by the vibration, thereby providing a glossy worked surface of a high accuracy.
In the above prior art locally pressurizing type injection molding machine, a molded product is ejected from the movable die 2 by an ejector device (not shown). Accommodation of an actuator or actuators for the stationary and movable punches 6 and 7 in addition to the ejector device, results in a complicated structure and an increase in size of the injection molding machine.